Digital video capabilities can be incorporated into a wide range of devices, including digital televisions, digital direct broadcast systems, wireless communication devices, personal digital assistants (PDAs), laptop computers, desktop computers, digital cameras, digital recording devices, cellular or satellite radio telephones, and the like. These and other digital video devices can provide significant improvements over conventional analog video systems in creating, modifying, transmitting, storing, recording and playing full motion video sequences.
A number of different video encoding standards have been established for communicating digital video sequences. The Moving Picture Experts Group (MPEG), for example, has developed a number of standards including MPEG-1, MPEG-2 and MPEG-4. Other standards include ITU H.263, QuickTime™ technology developed by Apple Computer of Cupertino Calif., Video for Windows™ developed by Microsoft Corporation of Redmond, Wash., Indeo™ developed by Intel Corporation, RealVideo™ from RealNetworks, Inc. of Seattle, Wash., and Cinepak™ developed by SuperMac, Inc. These and other standards, including standards yet to be developed, will continue to evolve.
Video encoding standards achieve increased transmission rates by encoding data in a compressed fashion. Compression can reduce the overall amount of data that needs to be transmitted for effective transmission of image frames. The MPEG standards, for example, utilize graphics and video compression techniques designed to facilitate video and image transmission over a narrower bandwidth than could be achieved without the compression. In particular, the MPEG standards incorporate video encoding techniques that utilize similarities between successive image frames, referred to as temporal or interframe correlation, to provide interframe compression. The interframe compression techniques exploit data redundancy across frames by converting pixel-based representations of image frames to motion representations. In addition, the video encoding techniques may utilize similarities within image frames, referred to as spatial or intraframe correlation, in order to achieve intra-frame compression in which the motion representations within an image frame can be further compressed. The intraframe compression is typically based upon conventional processes for compressing still images, such as discrete cosine transform (DCT) encoding.
To support the compression techniques, many digital video devices include an encoder for compressing digital video sequences, and a decoder for decompressing the digital video sequences. In many cases, the encoder and decoder comprise an integrated encoder/decoder (CODEC) that operates on blocks of pixels within frames that define the sequence of video images. In the MPEG-4 standard, for example, the encoder of a sending device typically divides a video image frame to be transmitted into macroblocks comprising smaller image blocks. For each macroblock in the image frame, the encoder searches macroblocks of the immediately preceding video frame to identify the most similar macroblock, and encodes the difference between the macroblocks for transmission, along with a motion vector that indicates which macroblock from the previous frame was used for encoding. The decoder of a receiving device receives the motion vector and encoded differences, and performs motion compensation to generate video sequences.
The video encoding process is computationally intensive. In particular, the process of comparing video blocks to previously transmitted video blocks requires large numbers of computations. Improved encoding techniques are highly desirable, particularly for use in wireless devices or other portable video devices where computational resources are more limited and power consumption is a concern.